Purpose. The visibility of a target stimulus can be affected by other stimuli presented on the parts of visual fields that is outside the target classical receptive field. We investigated the spatial properties of the non-classical receptive field psychophysicaly by observing how flankers at various locations affect contrast discrimination. Methods. In our dual-masking experiment, observers detected a 4 cy/deg vertical Gabor target superimposed on a vertical pedestal in the presence of either vertical or horizontal flanker Gabors. The distance to the collinear flankers varied from 1 to 7.5 wavelengths. Flanker locations were 0, 11, 22, 45 and 90 deg. away from the collinear axis. The pedestal contrast was from 1% – 50% with flanker contrast at 50%. We measured target thresholds with a 2IFC paradigm. Results. Compared with the no-flanker condition, the collinear flankers decreased target threshold at low pedestal contrasts (facilitation) and increased threshold at high contrasts (suppression) as expected from cat physiology (Chen et al., 2001, Neuroreport, 655–61). The size of low contrast facilitation increased with distance up to 4 wavelengths and decreased beyond that, while the high contrast suppression showed the opposite trend. For the vertical flankers, the greatest flanker effects (both facilitation and suppression) occurred at the collinear location and decreased monotonically as flanker location deviated from the collinear axis. For the horizontal flankers, the greatest flanker effect occurred at the 45 deg location. Conclusion. These flanker contrast effects are modeled with our sensitivity modulation model (Chen & Tyler, 2001, Proc. Roy. Soc. Ser. B., 509–16). The model suggests that the flanker effects are multiplicative terms applied to both the excitatory and inhibitory terms of a divisive inhibition response function. The model parameters show that the excitatory flanker effect is narrowly tuned in space while the inhibitory effect is broadly tuned.